1. for the sun to be stable, the inward and outward forces within the sun must be in equilibrium. part of the fusion reaction. focused in the core. balanced with temperature and density. 2. the parallax of a star is observed because all stars have the same apparent brightness. parallax increases with distance. stars do not move. the observer moves.

1)   For the sun to be stable, the inward and outward forces within the sun must be in equilibrium. Part of the fusion reaction is focused in the core and is balanced with temperature and density.

in other words, the outward pressure of gas heated by fusion is balanced by the inward pull of gravity, leaving the star in hydrostatic equilibrium. This balance of forces lasts for most of a star's life, maintaining its steady temperature. Radiation and convection carry the energy from the core out through a star's atmosphere.

2) Astronomers use an effect called parallax to measure the earths distance to nearby stars. Parallax is the apparent displacement of an object because of a change in the observer's paradigm.

As the Earth orbits the Sun, a nearby star will appear to move against the more distant background stars. Astronomers can measure a star's position once, and then again 6 months later and calculate the apparent change in position. The star's apparent motion is called stellar parallax.

newton's first law of motion